Conventional ethylene-based polymerization processes which employ a solvent are well known. Such conventional ethylene-based polymerization processes include, but are not limited to, solution polymerization process, slurry phase polymerization process, and combinations thereof but do not include gas phase or high pressure polymerization processes. Ethylene-based solution and slurry polymerization processes typically employ one or more loop reactors, isothermal reactors, pipe flow reactors, stirred tank reactors, batch reactors, in parallel or series, and/or any combinations thereof and typically utilize coordination catalysts. Following the polymerization reaction in ethylene-based polymerization reactions, the resultant mixture, e.g. ethylene/α-olefin interpolymer, and solvent stream is typically removed from the reactor and the interpolymer is isolated. After being used in the polymerization reaction, the solvent contains one or more of a number of polymerization by-products, including catalyst by-products, acid neutralizer and its by-products, water, hydrogen, ethylene and comonomer. Solvent is typically recovered via a solvent recovery unit, i.e. heat exchangers and vapor liquid separator drum. Following recovery, the solvent may be recycled back into the polymerization system. In some systems, the solvent is sent through a purification bed, which contains an adsorbent, to remove certain polymerization reaction by-products, particularly polar compounds, such as water, prior to being recycled to a polymerization reactor.
Adsorbents used in purification beds require regeneration following some use and therefore, two or more purification beds are used so that at least one regenerated purification bed is online, purifying a solvent stream, while a nearly saturated purification bed is taken offline for regeneration. When purification beds are swapped in this manner, a temporary catalyst efficiency dip is observed in some systems. One such adsorbent which, when used in a regenerated bed being brought online, causes a substantial catalyst efficiency dip is MOLSIV-13X, available from UOP, LLC. MOLSIV-13X is alkali aluminum silicate having the general formula, Nax[(AlO2)x(SiO2)y].zH2O, wherein it is not a hybrid with alumina.
The present invention provides a process for recycling the solvent used in an ethylene-based solvent polymerization process, and a system therefor, which results in a catalyst efficiency dip of less than or equal to 20% when purification beds are swapped.